Endstops. If the board thinks both min and max endstops are triggered, it wont move. use m119 to ask the controller what it thinks the endstops are doing

some less likely things that I have seen.

You dont have any stepper drivers plugged in.
The stepper drivers are all dead, or plugged in backwards, or off by one pin.
You have plugged 5v in to the 12v 5A plug on the ramps.
You haven’t turned on your reprap power supply. Controller board runs fine from usb, but no steppers or heaters work with out additional power.

Like Dust said in an earlier post - check stepper wiring and adjust current to stepper.

4 wires from steppers are in two pairs. Different manufacturers use different colours so you are best to test to find pairs. You can identify the pairs without a multimeter by simply holding 2 wires together (with the other two separated) and noting if the stepper motor shaft is harder to turn. If the 2 wires are a pair, the shaft is harder to turn than it is with the wires separated. Once you have identified one pair - the other two wires are your other pair! Connect one pair to 1A and 1B and the other pair to 2A and 2B on RAMPS. Use software to actuate the extruder and if it is still just clicking, turn the pot on the stepper driver clockwise 1/8th turn at a time till it turns. You are best to test the stepper without filament first. Once you have filament loaded, you will also need the hot end up to temperature for the extruder to work. If you find that the extruder runs backwards compared to what you are expecting, un-plug it from the RAMPS and turn it around.

Are you saying + / - does not matter with stepper motors as long as the pairs are together at each end of the connector the motor should turn?
Eg the connector might look like + - - + or - + + - or + - + - however any combinations will turn the stepper motor?

Once you have the wiring correct and the current from the stepper driver set properly, the two other things that might stop a stepper motor from turning is 1) being too heavily loaded or 2) being asked to accelerate too quickly. You will find that for a given current, loading and acceleration, the degree of microstepping will also affect the workability of a stepper motor. You configure microstepping by adding jumpers to the 3 pairs of pins underneath the stepper drivers on the RAMPS board.

The usual thing that causes blockages with PLA is heat transferring back up the filament above the melt zone. PLA goes soft and expands in the colder parts of the hot end and causes too much friction.

Things that help eliminate this are:
1. Use a hot end that has a PTFE tube liner (not an all-metal hot end). The PTFE liner can accommodate some expansion and is low friction.
2. Don't leave the printer sitting idle with the hot end heated up for any longer than you have to (You can set "auto extrude while idle" in firmware to periodically extrude if it is up to temperature and not printing).
3. Run the printer faster - faster thru-put of filament keeps the part above the melt zone cooler by introducing fresh filament faster.
4. Run with the extruder a little bit cooler.
5. Use a higher torque stepper to push the filament or increase the current if your existing stepper isn't being pushed too hard at the moment.
6. Run a continuous fan aimed at the hot end above the heater.

This last point will make your extruder very forgiving of other faults. I had blockages on my budaschnozzle when I first got it running. When I mounted a fan to cool the heatsink between hot and cold ends, the regular blockage problem went away completely.

Of course there are other causes of blockage. You could have accumulated dust or remnants of ABS blocking the nozzle. Any fragments of PTFE from installation of your bowden tube will cause blockages too. You may be able to clear blockages by running the hot end 20 degrees hotter than normal and forcing the filament thru by hand. Another approach is to disconnect the Bowden tube and heat the hot end up from cold while pulling the filament out. Before it gets to melting temperature it should pull free, hopefully pulling the entire filament out including the small diameter piece that was inside the nozzle hole. This should extract any contamination too. Another option - if you have a drill or wire that matches your nozzle size – is to try pocking it up the nozzle when it is hot.